Room scattering effects on the measured spatial distribution of delayed photofission neutrons from depleted uranium

Summary form only given. A series of experiments has been performed on the Mercury generator (8 MV, 200 kA, 50 ns) to investigate the use of a single, intense radiation pulse to induce photo fission. In these studies, a plate of depleted uranium (DU) is irradiated by the 8 MV-endpoint bremsstrahlung pulse produced by Mercury. Within this x-ray pulse, photons whose energies lie above the threshold of 5.26 MeV induce fission in the DU, producing about 2.7 neutrons per fission. The neutrons consist of two populations: prompt neutrons, which escape the DU within a few hundred ns of the bremsstrahlung pulse, and delayed neutrons, which continue to appear for minutes after the pulse. The results reported here pertain only to the delayed neutron population. These neutrons are unambiguously detected using an array of He-3 detectors deployed 1.0-4.5 m from the DU. However, the measured neutrons do not follow an inverse square law as a function of detector-to-DU distance. The farthest detector measured more than twice the counts, relative to the closest detector, that would be expected for an inverse-square distribution. Containment and scattering of the fission neutrons by the walls, floor, and ceiling of the Mercury chamber, and various nearby objects such as electrical boxes, are the likeliest explanation for the measured spatial distribution. Modeling of the experiment by the Monte Carlo transport code MCNPX supports this hypothesis. Results showing the influence of various features of the Mercury chamber, as well as their composition, are presented.